Raster graphics (or bitmap) images, such as Portable Network Graphics (PNG) files and Graphics Interchange Format (GIF) files, generated by most image-related programs are not transcoded particularly efficiently. For example, a significant percentage of PNG files are transcoded using Rgb24 or Rgba32 pixel formats, for which each pixel requires three or four bytes, respectively.
Thus, conventionally encoded raster images are somewhat inefficient with respect to transferring them over a network. As images become pervasive on the Internet, any efficiency improvement in the file size reduction of raster graphics is desirable.